Lactic acid is widely used in food, pharmaceutical and textile industries. It is also used as a source of lactic acid polymers which are being used as biodegradable plastics (Brown, S. F., 2003, Fortune, 148:92–94; Datta, R., et al., 1995, FEMS Microbiol. Rev. 16:221–231). The physical properties and stability of polylactides can be controlled by adjusting the proportions of the L(+)- and D(−)-lactides (Tsuji, F., 2002, Polymer 43:1789–1796). Optically pure lactic acid is currently produced by the fermentation of glucose derived from corn starch using various lactic acid bacteria (Carr, F. J., et al., 2002, Crit. Rev. Microbiol. 28:281–370; Hofvendahl, K. and Hahn-Hagerdal, B., 2000, Enz. Microb. Technol. 26:87–107). However, the fastidious lactic acid bacteria have complex nutritional requirements (Chopin, A., 1993, FEMS Microbiol. Rev. 12:21–38) and the use of corn as the feedstock competes directly with the food and feed uses.
Lignocellulosic biomass represents a potentially inexpensive and renewable source of sugars for fermentation (Duff, S. J. B. and Murray, W. D., 1996, Bioresource Technol. 55:1–33; Parajo, J. C., et al., 1996, Process Biochem. 31:271–280; Wyman, C. E., 1999, Ann. Rev. Energy Env. 24:189–226). The hemicellulose portion of biomass contains up to 35% of the total carbohydrate and can be readily hydrolyzed to monomeric sugars by dilute sulfuric acid (Saha, B. and Bothast, R. J., 1999, Appl. Biochem. Biotechnol. 76:65–77). With crop residues and hardwoods, this hemicellulose syrup contains primarily xylose. During acid hydrolysis, an assortment of microbial inhibitors is also produced which must be removed by treatment with lime (Amartey, S. and Jeffries, T., 1996, World J. Microbiol. Biotechnol. 12:281–283; Clark, T. A. and Mackie, K. L., 1984, J. Chem. Technol. Biotechnol. 34B:101–110; Martinez, A., et al., 2001, Biotechnol. Prog. 17:287–293).
Lactobacillus spp. are used extensively in industry for starch-based lactic acid production, the majority of which lack the ability to ferment pentose sugars such as xylose and arabinose (Carr, F. J., et al., 2002, Crit. Rev. Microbiol. 28:281–370). Although, Lactobacillus pentosus, Lb. brevis and Lactococcus lactis ferment pentoses to lactic acid, pentoses are metabolized using the phosphoketolase pathway which is inefficient for lactic acid production (Garde, A., et al., 2002, Bioresource Technol. 81:217–223; Tanaka, K., et al., 2002, Appl. Microbiol. Biotechnol. 60:160–167). In the phosphoketolase pathway, xylulose 5-phosphate is cleaved to glyceraldehyde 3-phosphate and acetyl-phosphate. With this pathway, the maximum theoretical yield of lactic acid is limited to one per pentose (0.6 g lactic acid per g xylose) due to the loss of two carbons to acetic acid.